From: "Frank A. Cancio Bello" <frank@generalsoftwareinc.com>
To: Steven Rostedt <rostedt@goodmis.org>,
Ingo Molnar <mingo@redhat.com>, Jonathan Corbet <corbet@lwn.net>,
linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
Cc: joel@joelfernandes.org, saiprakash.ranjan@codeaurora.org
Subject: [RFC 1/2] docs: ftrace: Clarify the RAM impact of buffer_size_kb
Date: Wed, 13 Nov 2019 11:32:36 -0500 [thread overview]
Message-ID: <0e4a803c3e24140172855748b4a275c31920e208.1573661658.git.frank@generalsoftwareinc.com> (raw)
In-Reply-To: <cover.1573661658.git.frank@generalsoftwareinc.com>
The current text could mislead the user into believing that the number
of pages allocated by each CPU ring buffer is calculated by the round
up of the division: buffer_size_kb / PAGE_SIZE.
Clarify that the number of pages allocated is the round up of the
division: buffer_size_kb / (PAGE_SIZE - BUF_PAGE_HDR_SIZE). Add an
example that shows how the number of pages allocated could be off by
5 pages more compared with how the current text suggests it should be.
Suggested-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Frank A. Cancio Bello <frank@generalsoftwareinc.com>
---
Documentation/trace/ftrace.rst | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)
diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index e3060eedb22d..ec2c4eff95a6 100644
--- a/Documentation/trace/ftrace.rst
+++ b/Documentation/trace/ftrace.rst
@@ -188,8 +188,17 @@ of ftrace. Here is a list of some of the key files:
If the last page allocated has room for more bytes
than requested, the rest of the page will be used,
making the actual allocation bigger than requested or shown.
- ( Note, the size may not be a multiple of the page size
- due to buffer management meta-data. )
+
+ The number of pages allocated for each CPU buffer may not
+ be the same than the round up of the division:
+ buffer_size_kb / PAGE_SIZE. This is because part of each page is
+ used to store a page header with metadata. E.g. with
+ buffer_size_kb=4096 (kilobytes), a PAGE_SIZE=4096 bytes and a
+ BUF_PAGE_HDR_SIZE=16 bytes (BUF_PAGE_HDR_SIZE is the size of the
+ page header with metadata) the number of pages allocated for each
+ CPU buffer is 1029, not 1024. The formula for calculating the
+ number of pages allocated for each CPU buffer is the round up of:
+ buffer_size_kb / (PAGE_SIZE - BUF_PAGE_HDR_SIZE).
Buffer sizes for individual CPUs may vary
(see "per_cpu/cpu0/buffer_size_kb" below), and if they do
--
2.17.1
next prev parent reply other threads:[~2019-11-13 16:32 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-13 16:31 [RFC 0/2] docs: ftrace: Clarify the RAM impact of buffer_size_kb Frank A. Cancio Bello
2019-11-13 16:32 ` Frank A. Cancio Bello [this message]
2019-11-13 16:37 ` [RFC 1/2] " Steven Rostedt
2019-11-14 20:20 ` Joel Fernandes
2019-11-14 21:36 ` Steven Rostedt
2019-11-15 4:24 ` Frank A. Cancio Bello
2019-11-15 13:30 ` Steven Rostedt
2019-11-15 15:59 ` Frank A. Cancio Bello
2019-11-15 16:03 ` Steven Rostedt
2019-11-13 16:33 ` [RFC 2/2] ** do not apply this patch ** Just for illustration purposes Frank A. Cancio Bello
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